Expander in a pulsation tube cooling stage

ABSTRACT

In an expander in a pulse tube cooling stage of a cooling system which pulse tube cooling stage comprises a pressure generator, a pulse tube cooling unit connected to the pressure generator and including a regenerator and a pulse tube with a heat exchanger arranged therebetween and a buffer volume in communication with the pulse tube and an expander arranged functionally between the pulse tube and the buffer volume, the expander includes capillary flow passages consisting of a material with high heat conductivity for dissipating heat from the gas flowing through the capillary flow passages.

[0001] This is a Continuation-In-Part application of international application PCT/EP01/13683 filed Nov. 24, 2001 and claiming the priority of German application 100 61 379.9 filed Dec. 9, 2000.

BACKGROUND OF THE INVENTION

[0002] The invention relates to an expander for a pulse tube cooling system comprising a pulse tube cooling stage or more co-operating pulse tube cooling stages and each pulse tube cooling stage comprises in principle a compressor, a pulse tube cooler consisting of a regenerator and a pulse tube with a heat exchanger disposed therebetween, a heat exchanger at the exit of the pulse tube and an expansion container connected thereto.

[0003] A pulse tube cooler is based on the known Stirling process wherein a gas is compressed and expanded in a cycle. The process has the advantage that there are no moving parts in the cold part of the cycle. This permits the use of a relatively simple design and provides for high operational reliability. Furthermore, there are only small mechanical vibrations. For temperatures down to 20° K, single stage arrangements may be used. Lower temperatures can be reached by the use of multiple stages.

[0004] Various types of such coolers are known in the art. Each pulse tube cooler type comprises a compressor capable of generating a cyclic gas flow, which is supplied to a regenerative heat exchanger—known as regenerator. From there, the gas flows through the pulse tube with the heat exchanger-cooling stage at its cool end and an expander operating at ambient temperature at the other end. The expander is a device in which acoustic energy associated with the pulsing gas flow is removed from the pulse tube. The main part of the cooling energy is provided by the flow of mechanical energy, which is part of the pulsed gas flow. The expander must transfer this mechanical energy flow from the pulse tube wherein the mechanical energy flow is converted to heat, to the environment.

[0005] It is known that, in order to obtain an efficient cooler, the expander must operate in such a way, that, at the end of the pulse tube, the phase angle of the periodic pressure wave is ahead of the phase angle of the volume flow.

[0006] Preferred phase angles are in the range of 30 to 60° . To achieve this, various methods are known. The three most important methods will lead to the invention, which is described further below. These three methods, their operations and efficiencies are therefore shortly described.

[0007]FIG. 9 shows schematically a pulse tube cooler based on the known principle, called double inlet system. The pressure wave generator 2 may be a piston compressor or any other type of compressor with separately controlled inlet and outlet valves. The oscillating gas flow is conducted by way of the connecting line 3 to the refrigeration generator 1, mainly to the regenerator 4, which is arranged in series with the pulse tube 6. The low temperature heat exchanger 5 is disposed between the regenerator 4 and the pulse tube 6 and the high-temperature heat exchanger 7 is disposed at the other end of the pulse tube 6. A second line 10 with a flow restrictor 11, the throttle impedance, provides for communication between the heat exchanger that is, its warm end, and the gas container 12. A part of the gas flow generated by the pressure wave generator 2 is branched off the connecting line and conducted, by way of the bypass line 8, which includes a flow restrictor 9, to the second line 10 to which it is connected between the heat exchanger 7 and the flow restrictor 11. The gas flow at the warm end of the pulse tube can be considered to be composed of two components, the so-called bypass flow through the bypass line 8 and the so-called throttle flow through the throttling section 11. The two gas flows differ in their amplitudes; their phase relationship can be adjusted so as to obtain optimal flow conditions at the warm end of the pulse tube. The expansion energy is dissipated in the throttling impedance provided by the restrictor 11.

[0008] This method however suffers from the fact that it is very difficult to suppress the detrimental time-averaged current, that is, the so-called DC current in the bypass branch 8. The line to which the lead line of the reference number 13 extends symbolizes the vacuum section. The components within that section are at low temperature whereas all other components are at ambient temperature.

[0009]FIG. 10 shows schematically another known pulse tube cooler. In this case, the expander is in the form of a so-called inertia tube phase shifter. This part consists of the conduit 10 a with circular cross-section, which extends between the pulse tube 6 and the buffer container 12 a. Its function is based on the inertia of the gas column, which oscillates in the conduit 10 a. But, because of the small mass of the gas, such arrangements must be operated either at a relatively high frequency or they must have long lines for low-frequency systems which are needed for obtaining very low temperatures.

[0010]FIG. 11 shows a third arrangement of a conventional pulse tube cooler. In this case, the oscillating gas flow is generated by a rotary piston compressor 2 a with valves 16 a to 16 d which are mounted in the suction line 14 and the supply line 15. The gas flow, which is supplied to the regenerator 4 by way of the supply line 3, is controlled by the valves 16 a and 16 b and the gas flow through the supply line 10 b to the pulse tube 6 is controlled by the valves 16 c and 16 d. Also, in this case, it is very difficult to provide for valve switching times such that optimum conditions for the cooler are provided.

[0011] As mentioned earlier, the known pulse tube coolers cannot provide sufficient phase shifting unless they have gas flows controlled by four valve pulse tube coolers. They require a well adjusted superimposition of two gas flows (two-inlet pulse tube cooler) or they do not operate at low frequency (inertia phase shifter). In addition, they need components, which provide for a continuous transition from the large flow cross-section present in the pulse tube to the small cross-section of the attached connecting lines. This function is included in the heat exchanger 7.

[0012] It is the object of the present invention to provide a cooling system without these disadvantages of the conventional systems.

SUMMARY OF THE INVENTION

[0013] In an expander in a pulse tube cooling stage of a cooling system which pulse tube cooling stage comprises a pressure generator, a pulse tube cooling unit connected to the pressure generator and including a regenerator and a pulse tube with a heat exchanger arranged therebetween and a buffer volume in communication with the pulse tube and an expander arranged functionally between the pulse tube and the buffer volume, the expander includes capillary flow passages consisting of a material with high heat conductivity for dissipating heat from the gas flowing through the capillary flow passages.

[0014] The invention is based on the fact that an oscillating gas flow is maintained at a constant temperature. As a result, heat is transferred to the surrounding medium during compression and heat is retrieved from the surrounding medium during expansion of the gas in the conduit. This process causes a phase shift between the oscillating pressure and the volume flow.

[0015] The parallel conduits need to be so dimensioned, that is they must have such a length and open width, that the expansion energy—the part responsible for the cooling—is converted by way of friction into the heat flow to be transferred to the surrounding medium. The flow within each capillary must therefore by isotherm. As a result, the diameter of the capillaries must be small in comparison with the thermal depth of penetration into the gas.

[0016] The isothermal pulsating gas flow in narrow conduits can be described by differential equations of the same type as they are used for electrical transmission lines subjected to losses. Therefore, an arrangement of parallel lines can be provided in such a way that they act like transmission lines, in which the inductive effect dominates the capacitive effect. With the capillary or the bundle of capillaries and the gas buffer volume as final impedance, the entrance impedance of such an arrangement causes the pressure wave to precede the volume flow as it is required for the withdrawal of the expansion energy from the pulse tube.

[0017] The impedance adaptation is achieved by the fact that in each capillary there is a sufficiently large friction resistance for the gas flow and a sufficiently large heat exchange through the capillary wall. To this end, the inner and outer surface areas of the capillaries must be large enough for conducting this heat away.

[0018] In a modification of the invention, an arrangement using, instead of a large number of parallel capillaries, a rod of a porous sintered material or a fleece or a stack of net-like discs, possibly in a compressed form. The components used should have good heat conductivity and are surrounded by a wall or a sheet, which also has good heat conductivity. The structure must have, over its cross-section and length, a suitable flow resistance and must be able to transfer the heat flow to the ambient medium as required. Such a rod can be described for dimensioning with an arrangement of sufficient quality by an equivalent bundle of capillaries. In order to have a continuous transition at the pulse tube exit, the cross-section of the rod should be of equal size, or the transition must at least be conical.

[0019] The pulse tube cooler may be further optimized if the pressure generator or compressor is connected to the pulse tube cooler by way of a conduit which originates from the inlet opening of the regenerator and includes two branches ending in the pressure generator and each provided with a control valve.

[0020] For fine-tuning a small control volume is arranged in the buffer volume, which small control volume is adjustable and is not compressible, whereby the buffer volume can be continuously adjusted within certain limits (FIG. 3). The small control volume may be a hydraulically operated piston or a solid material piston, which is insertable into, and removable from, the buffer volume.

[0021] If the surface is not sufficiently large for the transfer of heat to the ambient medium, the heat exchanger may be surrounded by a gas and liquid-tight housing with an inlet and an outlet connected to a cooling circuit (FIG. 4).

[0022] If the capillary flow channel or the enclosed and sintered rod extend into the buffer volume, the wall of the buffer volume must consist of a material with good heat conductivity in order to permit a sufficient heat flow to the ambient medium (FIG. 5). The final impedance, the expander, then consists in principle of the unit, which forms the heat exchanger at the pulse tube outlet, and at the same time, the connection to the buffer volume and also of the buffer volume.

[0023] The buffer volume can be connected to the pressure generator by way of a line, which includes a dosing valve (FIG. 7). This permits to provide a time-averaged net mass flow-through the pulse tube.

[0024] The unit providing for the heat dissipation to the ambient medium and also for the conduction of gas between the pulse tube and the buffer volume consists of parallel capillaries or sintered sponge-like or wool or felt-like materials with good heat conductivity. If the capillaries are evenly distributed over the cross-section at the warm end of the pulse tube or the materials have there the same cross-section, an additional flow controller as used in the state of the art is not needed. In this case, the flow controller is the device, which ensures a uniform flow with uniform flow speed over the whole cross-section. The heat exchanger at the warm end provides for the flow direction.

[0025] Such arrangements are very compact even with frequencies in the range of 2 Hz. In spite of the relatively small size, the arrangements have a large area for the heat transfer. In addition, the time-averaged net mass flow through the pulse tube can be interrupted or controlled if desired.

[0026] The invention will be described below in greater detail on the basis of the accompanying drawings comprising FIGS. 1-8. FIGS. 9-11 are provided for an explanation of the principle involved.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027]FIG. 1 shows a pulse tube cooler with capillary flow channels installed therein,

[0028]FIG. 2 shows the valve-controlled supply line from the pressure generator to the regenerator,

[0029]FIG. 3 shows the buffer volume with a controllable expander installed therein,

[0030]FIG. 4 shows an arrangement for a forced heat removal at the expander,

[0031]FIG. 5 shows the expander arranged so as to extend directly into the buffer volume,

[0032]FIG. 6 shows a sintered rod as an expander,

[0033]FIG. 7 shows an arrangement in which the buffer volume can be selectively connected to the pressure generator,

[0034]FIG. 8 shows a cooling arrangement with a two-stage setup of pulse tube coolers,

[0035]FIG. 9 shows the general known arrangement of a pulse tube cooler with a flow resistance in the connecting line to the buffer volume and in the bypass line,

[0036]FIG. 10 shows a conventional pulse tube cooler in its most simple form for an explanation of the principle, and

[0037]FIG. 11 shows a conventional pulse tube cooler with connecting lines between the pressure generator and the regenerator and a connecting line between the pressure generator and the heat exchanger at the warm end of the pulse tube each provided with two control valves.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0038] The expander 20 shown in FIG. 1 is so designed that a phase shift between the pressure and the volume flow at the warm end of the pulse tube 6 c can be established which is optimal for the cyclic process. In this way, a pulse tube cooler of high efficiency and simple design can be provided. New herein is the arrangement of narrow flow channels or capillary passages 20 between the outlet of the pulse tube 6, that is, the warm end thereof, and the gas container, that is, the buffer volume 21, which is dimensioned in accordance with the output, that is, the size, of the pulse tube cooler. It is important that the flow channels 20 consist of a material with good heat conductivity to the ambient medium, which is the cooling medium.

[0039]FIG. 2 shows schematically a preferred arrangement of a single-stage pulse tube cooler. The physical process steps are described below in detail on the basis of this arrangement:

[0040] The single stage pulse tube cooler is operated by a rotary piston gas compressor 2 a with the intake connection 14 and the supply connection 15. The two tubular connections 14 and 15 include each a valve 16 a and respectively, 16 b for closing and opening the connection 14 and 15 respectively. By way of a T member 16 c, the connections 14 and 15 are joined and connected by way of line 3 to the refrigeration head 1 or the cold end of the cooling arrangement, that is, more accurately, to the inlet of the regenerator 4.

[0041] The refrigeration head 1 of the pulse tube refrigeration apparatus comprises the regenerator 4, a heat exchanger 5 at the cold end, a pulse tube 6 c and a final impedance connected at the exit of the pulse tube and extending into the surrounding medium. The final impedance comprises a large number of capillaries 20 connected with one of their ends to the exit of the pulse tube 6 and, with their opposite ends to a buffer volume in the form of a gas container 21.

[0042] The regenerator 4 comprises a stack of porous materials having a high specific heat capacity, preferably of formed stainless steel lattice discs stacked on top of one another in a cylindrical housing. The pulse tube 6 is a cylindrical tube filled with a refrigeration medium, typically helium, which is maintained at a pressure oscillating by several bars about an average pressure of about 20 bar. The heat exchanger 5 is the component by which the low temperature generated by the oscillating internal gas flow is transferred to the outside user by way of a heat transfer medium which is not shown herein. In addition, the heat exchanger 5 acts as a flow controller or distributor such that the flow is evenly distributed over the cross-section of the entrance area of the pulse tube 6.

[0043] The refrigeration effect is achieved by the cyclical process with the following steps for the gas within the pulse tube:

[0044] A. Compression

[0045] By opening the discharge valve 16 a, pressurized hot gas is conducted through the regenerator where it is cooled and then flows through the heat exchanger 5 into the pulse tube 6. Another gas stream from the buffer volume 21 enters the pulse tube 6 through the capillaries 20 at the warm end of the pulse tube 6. For this to occur, the buffer volume 21 and the capillary 20 must be so dimensioned that a resonance with a corresponding phase shift between the two gas flows entering and then again leaving the pulse tube occurs.

[0046] B. Shift Toward the Warm End

[0047] After a certain time corresponding to the gas flow entering at the cold end, the pressure in the pulse tube 6 becomes higher than the pressure in the buffer volume 21 and the gas having a temperature higher than the ambient medium at the warm end of the pulse tube 6 flows back through the capillaries 20 into the buffer volume 21. During this step, heat is dissipated from the capillaries to the ambient medium.

[0048] C. Expansion

[0049] At this point the discharge valve 16 a is closed and the intake valve 16 b is open. At the beginning of this step, gas is discharged through both ends of the pulse tube 6. The pressure and temperature in the pulse tube 6 drop.

[0050] D. Shift Toward the Cold End.

[0051] Finally, a gas stream from the buffer volume 21 enters the warm end of the pulse tube 6 and, at the same time, cold gas flows back to the regenerator 4. During this step, heat is absorbed by the cold gas flow in the heat exchanger 5.

[0052] Expressed differently, the continuous cycle can be described as a process wherein the gas in the pulse tube acts like a piston which, in a mechanical way, transfers the expansion energy from the cold end of the pulse tube to the warm end where it is dissipated in a heat flow transferred d to the ambient medium around the capillary bundle. To this end, the warm end of the pulse tube must be closed by a well-adopted flow impedance that is a component which must fulfill certain main conditions:

[0053] controlled resonance

[0054] adapted flow resistance

[0055] to convert energy to heat, and

[0056] establishing resilient flux in the pulse tube.

[0057] In accordance with such different functions, different designations for the component may be selected such as pulse tube expander, thermal phase shifter, or inductively effective pulse-tube end-impedance.

[0058] The numeric examination of such pulse tube end systems show that this type of phase shifter is most advantageous for coolers, which operate at low frequencies. As an example, a pulse tube cooler is analyzed to increase 50W from 50K to 300K. The respective pulse tube has a diameter of about 45 mm and is 200 mm long. The respective expander comprises about 40 capillaries each with an inner diameter of 0.3 mm and a length of 150 mm. The gas container of the buffer volume must have about 200 cm³.

[0059] Further possible components are:

[0060] The gas container or buffer volume 21 in FIG. 3 is provided with the volume 33, which, within limits, is continuously adjustable. It is for example in the form of a hydraulic or solid piston provided with a fluid supply line 23 for changing the buffer volume, that is for finely adjusting the impedance volume to the system.

[0061]FIG. 4 shows an expander 20 with forced cooling which for that purpose is enclosed in a tube 25 with connections 26 a and 26 b for conducting a coolant past the expander capillaries 20.

[0062] In FIG. 5, the capillaries 20 are shown arranged within the buffer volume 21. In order to provide in this case a sufficiently good heat flow path to the ambient medium the wall of the buffer volume at least must consist of a material with good heat conductivity.

[0063] An alternative solution which is considered to be less effective than a bundle of parallel capillaries is a sintered rod of heat conducting material with a suitable pore size or metal/stainless steel wool, possibly with a felt-like structure. Since the pores or passages are uniformly distributed over the whole surface of this material and statistically uniformly distributed flow passages are provided, the rod must be enveloped between the exit of the pulse tube and the entrance of the buffer volume in a gas-tight manner by an envelope with good heat conductivity so that only flow channels are formed leading from the entrance area at the pulse tube exit to the exit area at the buffer volume. As in the capillary bundle, the entrance area should be the same as the exit area and the entrance area should be connected directly to the pulse tube and have the same diameter (FIG. 6).

[0064] If a net mass flow is to be established which is different from zero, a conduit 24 can be installed between the buffer volume 21 and the pressure generator with a control valve 25 arranged in the conduit 24 for continuously adjusting the flow cross-section. (FIG. 7) If the conduit 24 is not directly connected to the pressure generator, it should be connected to the suction line 15 between the pressure generator 20 and the respective valve 16 b.

[0065] A two-stage pulse tube cooler is shown for example in FIG. 8. The pre-stage comprises a vacuum area 13 including the regenerator 4 a, the pulse tube 6 a and the heat exchanger 5 a disposed therebetween. The respective expander 20 a consisting of heat exchanger, capillary bundle and buffer volume extends into the ambient medium and is connected to the pressure generator 2 a by way of the conduit 24 with the control valve 25 a for providing a specific net volume flow adjustment. The second stage comprises, in the vacuum container 13, the regenerator 4 b, the pulse tube 6 b and the associated heat exchanger 5 b. The associated expander 20 b is in principle the same as the expander 20 a of the pre-stage and also extends into the ambient medium. The housing of the expander may be provided with heat transfer means such as ribs to improve the transfer of heat to the ambient medium. It too is connected, by way of the control valve 25 b for a specific volume flow adjustment for the second stage, to the pressure generator 2 a. The pressure generator of the second stage is formed by the connecting conduit from the regenerator 4 a of the first stage to the pulse tube 6 a of the first stage. This connecting conduit and the corresponding one of the second stage together with the heat exchangers 5 a and 5 b form in the vacuum container 13 the heat sinks used in the system. 

What is claimed is:
 1. An expander in a pulse tube cooling stage in a cooling system, said pulse tube cooling stage comprising essentially a pressure generator, a pulse tube cooling unit connected to said pressure generator and including a regenerator (4), a pulse tube (6) with a heat exchanger (5) arranged therebetween and a buffer volume (21) in communication with said pulse tube (6) and an expander (20) arranged functionally between the pulse tube (6) and the buffer volume (21), said expander (20) including capillary flow passages in a material with high heat conductivity.
 2. An expander according to claim 1, wherein said expander comprises a plurality of capillary tubes of equal length and diameter.
 3. An expander according to claim 1, wherein said expander consists of a rod of a sintered, porous, heat-conductive material disposed in a gas and liquid tight housing.
 4. An expander according to claim 1, wherein said pressure generator includes an inlet and an outlet and is connected to said regenerator (4) by way of a line including a first branch (14) connected to the inlet and a second branch (15) connected to the outlet, each branch including a control valve (16 a, 16 b).
 5. An expander according to claim 4, wherein said buffer volume (21) includes a control volume which is separated from the buffer volume in a gas and liquid tight manner and which is non-compressible and by which the buffer volume is adjustable.
 6. An expander according to claim 4, wherein said expander (20) is disposed in a housing connected to a cooling circuit for cooling the expander (20).
 7. An expander according to claim 5, wherein said expander extends at least partially into said buffer volume 21 and the buffer volume walls consist of a material with high heat conductivity.
 8. An expander according to claim 6, wherein said buffer volume is connected to the pressure generator (2 a) by way of a line (24) including a control valve (25).
 9. An expander according to claim 1, wherein said buffer volume wall is provided with surface increasing means of heat conductive material for dissipating heat from said buffer. 